Sensor apparatus

ABSTRACT

A measurement head particularly for borehole use can measure multiple parameters with no electronics in the head and only a single conductor cable. Double-ended tuning forks ( 26, 28  and  38, 40 ) in the head are arranged to respond to different parameters such as temperature and pressure by adjusting their resonant frequencies. A drive signal on the conductor ( 50 ) is applied to transducers ( 30, 32, 42, 44 ) on all the tuning forks and those whose instantaneous resonant frequency is close will resonate. The signal is removed and the transducers return a decaying signal at the resonant frequency along the conductor. Other drive frequencies are tried, to locate the other sensors whose frequency ranges are separate.

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing of, and claimspriority to, International Application No. PCT/GB01/01326, filed Mar.27, 2001, which in turn claims priority to Great Britain PatentApplication No. 0007325.4 filed on Mar. 27, 2000 in Great Britain. Thecontents of the aforementioned applications are hereby incorporated byreference.

FIELD OF THE INVENTION

The present application relates to sensor apparatus. It seeks to addressthe long term reliability of such sensors when operated in adverseenvironments such as elevated temperatures or high radiation levels. Theapplication is applicable (inter alia) to the measurement of physicalparameters such as pressure, temperature and force, especially in hightemperature environments.

BACKGROUND TO THE INVENTION

It is often desirable to measure pressure, temperature and force inenvironments that have very high ambient temperatures, such as at thebottom of oil or gas wells, inside nuclear reactors, or inside jetengines. Many of these environments are also characterized byinaccessibility, and the need for extreme reliability of operation. Forexample, the pressure and temperature at the bottom of an oil or gaswell can be used to monitor the performance of the hydrocarbonreservoir. This information can aid the management of the production ofthat reservoir to maximise returns. The high temperatures that are oftenencountered at the bottom of oil or gas wells accelerate the aging andfailure of electronic devices that are typically used in pressure andtemperature gauge instrumentation.

As it is very costly to shut in an oil or gas well to change thedownhole instrumentation, it is very important to maximize the lifetimeand reliability of this instrumentation in this extreme environment. Onesolution is to install pressure and temperature gauges with noelectronics. Strain gauge pressure sensors and platinum resistancethermometers are well known and extremely reliable at high temperatures.However, they require multiple insulated electrical conductors from thesurface to the downhole sensors. It is difficult and expensive to obtainmultiple conductor cable that is reliable in an oil or gas well, and themultiple pin connectors that are consequently required in theinstallation are also a source of high cost and potential unreliability.

One solution to measuring pressure at high temperatures, through asingle conductor cable, is given in U.S. Pat. No. 4,255,973. In thissensor, a vibrating wire is connected to a bellows that is open to thefluid medium whose pressure is to be measured. Variations in pressurecause the tension in the wire to change, and hence the natural resonantfrequency of this wire. The wire is placed in a magnetic field, and acurrent from the single conductor cable is passed through the vibratingwire. A remotely located positive feedback amplifier is connected to theother end of this single conductor cable and produces oscillations atthe resonant frequency of the wire. Whilst this method admirablyeliminates electronics from the sensor, it only enables one sensor to beactivated per cable. Accordingly, a multiple parameter or multiplelocation sensor will still require several cables. In addition, thesystem does not appear well adapted for long distance use.

SUMMARY OF THE INVENTION

The present invention therefore provides a sensor apparatus comprising aplurality of resonant sensors each of whose operating frequencies shiftswithin a range dependent on at least the parameter sensed by therespective sensor, the ranges of each of the sensors being different.Thus, each can be interrogated by a signal along the same conductorenabling the use of a single conductor cable. It is of course preferredthat the frequency ranges of the sensors are non-overlapping.

The sensors preferably comprise a vibrating element whose resonantfrequency depends on the parameter measured by that sensor. Thisdependency can be obtained if the sensors further comprise a means forexerting a physical force on the vibrating element dependent on theparameter.

Where the parameter is temperature, the physical force can be exerted byholding the vibrating element of that sensor by a member of a materialwith a different coefficient of thermal expansion.

Where the parameter is pressure, the physical force can be exerted byholding the vibrating element of that sensor at one end by a memberwhich is exposed to the relevant pressure and adapted to exert a forcein response thereto.

The vibrating element(s) can be driven by a transducer such as a piezoelement. The transducer can be driven by an oscillating signal. Theywill in turn produce an oscillating voltage in response to anoscillation of the vibrating element. It is preferred that theoscillating signal is fed to all transducers via a single conductor.

The drive signal to the transducer can be applied intermittently. Thisallows a period during which the system can “listen” for a return signalwithout it being swamped by the drive signal. Thus, it is also preferredthat the system includes a signal analyser to detect signals from thetransducers.

It seems that although double-ended tuning fork are known as such forother purposes, their advantages in borehole use have never beenappreciated. In particular, their ability to permit the use of a singleconductor cable leading to a downhole gauge or measurement headcontaining no electronics does not appear to have been noted.Accordingly, in a further aspect the present invention provides a sensorapparatus for remote parameter measurement in boreholes comprising ameasurement head, a cable leading from the measurement head to remoteinstrumentation, a conductor within the cable leading to transducerswithin the measurement head which drive and are driven by resonatingelements also within the measurement head and whose resonant frequencyresponds to the measured parameter.

The present invention thus enables the sensing of multiple parametersover a long single conductor cable. According to the present invention,two or more resonant sensors can be connected to a common singleconductor cable. This overcomes the limitations of the prior art byproviding a single conductor sensor that can operate without electronicequipment. Previously, single conductor sensors required electronicequipment at the sensor in order to provide analysis and multiplexingfunctions. Such electronics is vulnerable and places a limitation on theequipment life and operating temperature. Sensors without electronicequipment have previously required multiple conductors.

Each resonant sensor may be set into mechanical vibration by anelectrical signal driven from a remote instrumentation package, thissignal having a frequency at, or close to, the resonant frequency of thesensor. Each resonant sensor is connected to the common cable and isdesigned to resonate at a range of frequencies that do not overlap withthe other sensors connected to the common cable. The precise frequencyof resonance for each sensor depends primarily on the value of thephysical parameter to be measured by that particular sensor. The remoteinstrumentation package can sweep the electrical drive signal over arange of frequencies and measure the frequency of the response from eachof the sensors as each sensor is excited by this electrical drivesignal. Each frequency is then converted to a value for the physicalparameter being measured by that sensor, using previously recordedcalibration information, as is well known.

As the present invention allows for multiple sensors, the value from onesensor can be used to perform an error correction on other sensors, in aknown fashion. Typically, the value from the temperature sensor is usedto apply a correction to (for example) a pressure sensor, as pressuresensors generally have a secondary response to variations in temperatureas well as their primary response to variations in pressure.

Once the remote instrumentation package has determined the approximatefrequencies of each of the sensors, a complete frequency sweep is notsubsequently required as the instrumentation package may perform areduced frequency sweep over a narrow range of frequencies close to thelast frequency response from each of the sensors. The frequency sweepmay be halted at the precise resonant frequency of each sensor, toenable accurate measurement of the frequency at that point.

A sensor with a Q value that is not too high will be excited at itsresonant frequency by a drive signal which is merely close. Thus,frequent stimulation at the last known resonant frequency should besufficient provided the parameter is not changing too swiftly. Ifdesired, the drive frequency can be chosen by an algorithm employinghistorical data to predict a likely value, such as by linear or otherextrapolation of a previous trend.

In a further embodiment of the present invention, the electrical drivesignal is periodically interrupted, i.e. switched on and off. While theelectrical drive signal is switched off, the remote instrumentationpackage can amplify the signal returned from the sensor, which willstill be resonating provided the electrical drive signal was close tothe resonant frequency prior to being switched off. This enables thesignal from the sensor to be detected over very long cables.

In a still further embodiment of the present invention, sensors may belocated at different physical locations and connected together by thecommon single conductor cable. For example, in an oil or gas wellseveral sensor packages may be located at different depths in the well,connected together by one single conductor cable.

Many types of electrically driven resonant sensors are known. Thevibrating wire sensor disclosed in U.S. Pat. No. 4,255,973 is oneexample. Double ended tuning forks (DETFs) are also widely used. In thistype of resonant sensor, each tine of the DETF vibrates in anti-phase sothat the sensor is balanced. This can provide a relatively high Q factorand reduce the effect of outside influences on the sensor. Theresonating element of the sensor is typically made of steel, althoughquartz and silicon have the well known benefits of low drift and veryhigh “Q” factor. Typically resonators are driven eitherelectromagnetically as in the case of U.S. Pat. No. 4,255,973, or usingpiezoelectric drivers known as PZTs, or the natural piezoelectric effectthat occurs in quartz crystals.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures, in which:

FIG. 1 shows a sectional view through a measurement head according tothe present invention;

FIG. 2 shows an oilwell including a sensor according to the invention;

FIG. 3 shows the remote instrumentation for the invention; and

FIG. 4 shows the voltage existing on the single conductor, with time.

DETAILED DESCRIPTION OF THE EMBODIMENT

FIG. 1 shows a pressure and temperature gauge for use in oil and gaswells. A cylindrical pressure housing 10 is electron beam welded to acablehead 12 at an upper end and a plug 14 at a lower end. In thisdescription, upper and lower refer to orientations when the device is inits normal operating state within a well. This forms a sealed chamberthat can withstand the high pressures required for this form of gauge.The fluid in the oil or gas well can enter via a pressure inlet 16 inthe plug 14 and into an expansion volume 18 enclosed by bellows 20sealed to the plug 14 at their lower end and to the base 24 of aclosed-ended cylinder 22 at their upper end.

A pair of tines 26, 28 form a double ended tuning fork that is driven tovibrate in antiphase by a corresponding pair of PZT devices 30, 32. Thefork thus formed is located within the cylinder 22, the upper end of thefork being attached to the inner face 34 of the upper end of thecylinder 22. The lower end of the fork is attached to a mounting 36 thatis itself secured to the plug 14. The mounting could be secured to thehousing 10, but securing the mounting 36 to the plug 14 allows easierassembly and servicing of the device. Slots are formed in the otherwiseclosed cylinder 22 to allow the mounting 36 to pass through.

The frequency of vibration of the tines 26, 28 is determined to someextent by temperature but also by the tension force to which they aresubject. As the external pressure of the fluid in the oil or gas wellincreases, the pressure in the expansion volume 18 is applied to thebase 24 of the closed-ended cylinder 22 and hence exerted on the upperend of the tines 26, 28 as these are secured to the mounting 36 at theirlower end. This increases the natural resonant frequency of the tines26, 28 with increasing pressure. Thus, the fork defined by tines 26, 28responds to pressure in the well.

A second double-ended tuning fork is defined by two further tines, 38,40. These are also driven to vibrate in antiphase by two further PZTdevices 42, 44. The second fork is again in a closed cylinder 46 whichis secured to the cablehead 12. Again, the cylinder 46 could be securedto the housing 10 but easier access is permitted by the designillustrated.

The cylinder 46 is made of a material with a higher thermal expansioncoefficient than the material from which the tines 42, 44 areconstructed. As a result, as the temperature increases the tension intines 42, 44 also increases, as does their natural resonant frequency.Thus, the second fork responds to the ambient temperature.

Tuning forks of this type have a natural response to temperature. Insome circumstances, therefore, it will be sufficient to rely on this andthe cylinder 46 will be unnecessary.

A cable 48 is secured to the cablehead 12 and consists of an outer metalsheaf sealed to the cablehead 12 to prevent the ingress of fluid. Asingle insulated electrical conductor 50 is contained within the cable48 and is connected to wiring 52 that leads to all four PZT devices 30,32, 42, 44.

FIG. 2 illustrates an oilwell 54 with cable 48 connected to a pressureand temperature gauge 8 constructed as described in relation to FIG. 1.At a remote location on the surface outside the oilwell 54, the cable 48is connected to instrumentation 56 which is in turn connected to acomputer 58 via a serial cable 60.

FIG. 3 shows the instrumentation 56 in more detail. The outer metalsheaf 62 of the cable 48 is connected to the instrumentation ground 64.This provides a return path for the electrical drive and the signalspassing on electrical conductor 50. A digital signal processor (DSP) 66generates a drive signal that is sent to an amplifier 68 and thence toconductor 50 via a switch 70 Switch 70 is also under the control of theDSP 66 via control line 72.

Any electrical signal returned from the conductor 50 can be amplified ina programmable gain amplifier (PGA) 74 before being digitized by ananalogue to digital converter (ADC) 76. The PGA 74 is under the controlof the DSP 66 and the digital results from the ADC 76 are passed to theDSP 66. Processed results from DSP 66 are passed in digital format tothe computer 58 via computer cable 60.

Referring now to FIGS. 1 and 3, the DSP 66 controls the sequence ofoperation. Firstly, switch 70 is closed and a drive signal at aparticular frequency is connected to conductor 50. This drive signaltravels to the PZT devices 30, 32, 42, 44. If the drive frequency isclose to the resonant frequency of a tine, they will be forced intooscillation. It is important to note that the resonant frequency of onlyone pair of tines, 26, 28 or 38, 40 can be close to any drive frequency,as the tines are designed so that no overlap of resonant frequencyoccurs, irrespective of the pressure or temperature. This is achievedeither by design of the physical aspects of the tines or by placing apreload on one or both pairs of tines. The latter is preferred as theprimary purpose of the temperature sensor is to calibrate the pressuresensor, in which case physically identical pairs of tines are preferred.

Next, switch 70 is opened and the signal from any resonating pair oftines is amplified in PGA 74 before being digitised in ADC 76 andanalysed by the DSP 66. The DSP 66 controls the gain of PGA 74 viacontrol line 78 to obtain an optimum signal level. The DSP 66 obtainsthe exact frequency of the signal by measuring the time between eachzero crossing of the signal, and taking an average. This exact frequencyis sent to the computer 58.

When the DSP 66 first starts operating, the drive frequency selected foreach interrogation sequence sweeps across the possible resonantfrequencies of first the pressure sensor and then the temperaturesensor, until the resonant frequency of each is found. This can takesome time, so thereafter the drive frequency selected when attempting toresonate the pressure sensor is the last measured resonant frequency ofthe pressure sense, from the previous sequence. A similar approach isadopted for the temperature sensor. This enables rapid interrogation ofthe sensors.

FIG. 4 further illustrates the sequence events in which a time trace ofthe voltage on the conductor 50 is shown. The trace shows the firstapplied drive frequency at 80, from the amplifier 68 with switch 70closed. The switch 70 is opened, and assuming the drive frequency wasclose to the resonant frequency of the pressure sensor there will be areturn signal 82 from the PZTs 30, 32 resulting from the continuedresonance of the tines 26, 28. A short gap 84 may be necessary to allowtransients from the drive signal to die away. The return signal will dieaway (as shown) as there is now no signal driving the resonatingelements.

The switch 70 is then closed again and the next applied drive frequency86 for the temperature sensor is applied for a time. The switch 70 isopened, and again, if the drive frequency was close to the resonantfrequency of the temperature sensor there will be a return signal 88 atthe resonant frequncy of the temperature sensor, after a short delay 90.

This pattern is immediately repeated with the applied drive frequency ineach case taken from the actual signal frequency on the previous cycle.The drive will then be close to the actual resonant frequency and willexcite the sensor to return a signal at that actual frequency. In thisway the applied drive frequency closely tracks the natural resonantfrequency.

In this example, the pressure sensor resonant frequency changes from4000 Hz to 5000 Hz over the pressure range 0 psi to 10,000 psi. Thetemperature sensor resonant frequency changes from 6000 Hz to 7000 Hzover the temperature range 0° C. to 200° C. Thus, the resonant frequencyranges do not overlap.

When the DSP 66 first powers up, it sweeps the frequency range 4000-5000Hz in 10 Hz steps, looking for the resonance from the pressure sensor.It then sweeps the range 6000-7000 Hz in 10 Hz steps, looking for theresonance of the temperature sensor. Once these resonances are found,the DSP 66 alternates between the last pressure resonance, and the lasttemperature resonance as shown in FIG. 4.

A simple linear calibration for pressure and temperature uses theformulae:Pressure (psi)=(F _(p)−4000)*10,000/1000 where F _(p) is the pressureresonant frequency in Hz;Temperature (° C.)=(F _(t)−6000)*200/100 where F _(t) is the temperatureresonant frequency in Hz.

For example, if the pressure resonant frequency were found to be 4500Hz, this would compute to a pressure of 5000 psi. If the temperatureresonant frequency were found to be 6500 Hz, this would compute to atemperature of 100° C. Of course, more complex calibration methods canbe applied, using polynomial equations to correct for any non linearityof the sensors, and using the temperature from the temperature sensor tocorrect the results from the pressure sensor. Such calibration methodsare well known.

1. A sensor apparatus comprising a plurality of resonant sensorsconnected to a common electrical conductor that provides drive signalsto each such sensor, wherein each sensor further comprises a vibratingelement, and wherein the resonant frequency of each sensor shifts withina range dependent on at least a parameter sensed by the respectivesensor, and wherein the ranges of the sensors being different andnon-overlapping.
 2. The apparatus according to claim 1 in which thevibrating element is driven by a transducer.
 3. The apparatus accordingto claim 2, wherein transducer is driven by an oscillating signal. 4.The apparatus according to claim 3, wherein the transducers produces anoscillating voltage in response to an oscillation of the vibratingelement.
 5. The apparatus according to claim 3 or claim 4, wherein theoscillating signal is fed to all transducers via the single electricalconductor.
 6. The apparatus according to any one of the claims 2 to 4 inwhich the transducer is a piezo element.
 7. The apparatus according toany one of the claims 2 to 4 in which the drive signal to the transduceris intermittent.
 8. The apparatus according to claim 7, furthercomprising a signal analyzer to detect signals from each of thetransducers.
 9. The apparatus of claim 1, wherein the plurality ofresonant sensors includes at least two sensors, each such sensor sensinga different parameter.
 10. A sensor apparatus comprising a plurality ofresonant sensors each of whose operating frequency shifts within a rangedependent on at least a parameter sensed by the respective sensor, theranges of the sensors being different and non-overlapping, and all ofthe sensors being supplied drive signals by a single electricalconductor.
 11. The apparatus according to claim 10, wherein each of thesensors comprises a vibrating element whose resonant frequency dependson the parameter sensed by that sensor.
 12. An apparatus comprising aplurality of resonant sensors connected to a common electrical conductorthat provides drive signals to each such sensor, wherein each sensorfurther comprises a vibrating element, and wherein the resonantfrequency of each sensor shifts within a range dependent on at least aparameter sensed by the respective sensor, the ranges of sensors beingdifferent, and wherein the sensors comprise a device for exerting aphysical force on the vibrating elements dependent on the parametersensed.
 13. The apparatus according to claim 12, wherein the parametersensed by at least one sensor is temperature and the vibrating elementof that sensor is held by a member of a material with a differentcoefficient of thermal expansion than that of the vibrating element. 14.The apparatus according to claim 12 or claim 13, wherein the parametersensed by at least one sensor is pressure and the vibrating element ofthat sensor is held at one end by a member which is exposed to pressurethat exerts a force on the vibrating element in response thereto.
 15. Asensor for remote parameter measurement in a borehole comprising ameasurement head, a cable leading from the measurement head to a remoteinstrumentation, a single electrical conductor within the cable leadingto transducers within the measurement head, each of which drives and isdriven by a resonating element within the measurement head and whoseresonant frequency responds to a measured parameter.
 16. An apparatus,comprising: a plurality of resonant sensors connected to a commonelectrical conductor that provides drive signals to each such sensor,wherein each sensor includes a double-ended tuning fork that comprises avibrating element, and wherein the resonant frequency of each sensorshifts within a range dependent on at least a parameter sensed by therespective sensor, the ranges of each of the sensor being different.